Populations and communities

All the individuals of the **same species** living in the **same area at the same time**.

All the populations of **different species** living together and **interacting** in the same area.

The **place** (the type of environment) where a species or community normally lives.

A **community** of organisms together with the **abiotic (non-living) environment** it interacts with.

The **number of different species** present in a community (a simple count, ignoring how many of each).

**Population → community → ecosystem** — one species, then many populations, then community plus its environment.

A community is the **living organisms only**; an ecosystem **also includes the abiotic (non-living) environment**.

Their populations **interact and depend on one another** — through feeding relationships, competition and other interactions.

**Autotrophs (producers)** that make their own food, and **heterotrophs (consumers and decomposers)** that take in food made by others.

An organism that **makes its own food**, usually by **photosynthesis** (a producer, e.g. grass or algae).

An organism that **takes in food made by other organisms** (a consumer or decomposer, e.g. a rabbit, fox or fungus).

An **ecosystem** — because it includes the non-living environment as well as the living organisms.

**No** — a community is living organisms only. Adding the abiotic environment makes it an **ecosystem**.

**Species richness** counts how many **different species** there are; **abundance** counts how many **individuals** of a species there are.

A **non-living**, physical or chemical feature of the environment (e.g. temperature, light, water, pH, salinity).

A **living** feature — an interaction with other organisms (e.g. food, competition, predation, disease).

Any of: **temperature, light intensity, water / rainfall, pH, salinity, dissolved oxygen, soil mineral nutrients**.

Any of: **food supply, competition, predation, disease, availability of mates**.

The **range of places where a species is found** — where its individuals actually live.

The range of values of an abiotic factor within which an organism can **survive**; outside it the organism is **absent**.

The factor is too extreme, so the organism **cannot survive there** and is **absent**.

The factor in **shortest supply** (or most extreme), which **holds back** growth or survival in that place.

**Iron** is scarce there, so even with plenty of light and nutrients, growth only increases when **iron is added** — iron is the limiting factor.

**Abiotic** (non-living conditions) **and biotic** (interactions with other organisms) — both together.

Through **competition, predation, disease or too little food** — a living factor can exclude a species even where conditions are right.

**Abiotic** — it is a non-living, physical condition.

**Biotic** — it is an interaction between living organisms.

Counting everything is **impractical** — there are too many, many **hide**, and many **move** — so a random **sample** is counted and scaled up.

To avoid **bias**, so the sample is **representative** of the whole habitat.

**Quadrat sampling** — count organisms in random quadrats, find the mean, and scale up.

**Capture–mark–release–recapture** — moving animals can't be counted in a fixed area.

Count organisms in several **random quadrats**, find the **mean per quadrat**, then **scale up** to the whole habitat area.

**Capture** and **mark** a first sample, **release** them, let them mix, **recapture** a second sample, and count how many are marked.

**N = (M × n) ÷ m**.

The **number marked** (and released) in the **first** sample.

The **total size of the second** sample (the recapture).

The number in the second sample that were **already marked** (recaptured marks).

The **estimated total population size**.

Marked animals **mix back evenly**; **no births, deaths or migration** between samples; marks are **not lost or harmful**; marking doesn't change the chance of recapture.

N = (60 × 80) ÷ 20 = **240**.

**Sit still → quadrat; runs away → recapture.**

The **maximum population size** of a species that a habitat can support over a long period, given its resources.

A **sigmoid (S-shaped) curve**: a slow lag start, a rapid exponential rise, then a plateau at the carrying capacity.

**Lag → exponential → transitional → plateau.**

There are **plenty of resources and few limiting factors**, so nearly all individuals survive and reproduce — the population grows by ever-larger amounts.

**Limiting factors** (shortage of food, water, space; disease; predation) raise deaths until **births ≈ deaths**, so growth stops at the carrying capacity.

**Births ≈ deaths** — they are roughly equal, so the population stays about the same size.

Any factor that **slows or stops** a population growing — e.g. shortage of food, water or space, disease, or predation.

One whose effect gets **stronger as the population becomes more crowded** — e.g. competition, disease or predation.

One that acts the **same regardless of population density** — e.g. drought, fire, flood or extreme cold.

A **limiting factor** such as competition for food / limited space, as the population reaches its carrying capacity.

There is an **optimum temperature**; temperature sets the rate of enzyme reactions (e.g. photosynthesis), so growth is fastest at the optimum, slow when too cold, and reduced when too hot (enzymes denature).

Resources (food, water, space) are **limited**, so as numbers rise, limiting factors take effect and growth slows to the carrying capacity.

A close interaction **between two different species** in a community.

By a **pair of signs** — for each species: benefits (**+**), harmed (**–**) or unaffected (**0**).

**Mutualism** — it is the only **+ / +** relationship.

**Interspecific competition** — it is the only **– / –** relationship.

An interaction in which **two species live together and both benefit** (+ / +).

An interaction in which **two species compete for the same limited resource**, so **both are harmed** (– / –).

An interaction in which one animal (the predator) **kills and eats** another animal (the prey). Predator +, prey –.

An interaction in which an **animal feeds on a plant**. Herbivore +, plant –.

An interaction in which a **parasite lives on or in a host**, gaining nutrients (+) while harming the host (–).

An interaction in which a **pathogen (a disease-causing organism) infects a host**, benefiting itself (+) and causing disease in the host (–).

A **predator kills its prey quickly**; a **parasite lives on/in one host** and feeds off it over time without quickly killing it.

Almost always **interspecific competition** (– / –).

**Name** the relationship **and justify** it using the **effect on each species** (the benefit or harm to each).

The **full role** of a species in its community — its abiotic tolerances, the resources it uses, and its interactions with other species.

Two species that need the **same limited resource** cannot coexist **indefinitely**; the better competitor excludes the other.

It is **excluded** — it dies out locally, or survives only by shifting to a **different niche** (using a different resource).

The **whole niche** a species could occupy if **no competitors** were present.

The **smaller** part of the niche a species **actually** occupies once competitors restrict it.

It shrinks the **fundamental niche** down to a smaller **realized niche**.

When a **plant releases a chemical** that **inhibits the growth/germination of other plants** nearby, reducing competition.

When a **microorganism releases a chemical** that **inhibits the growth of other microorganisms**, reducing competition.

The **black walnut** tree releases a chemical into the soil that stops many plants growing beneath it.

**Allelopathy** = a **plant** inhibits other **plants**; **antibiosis** = a **microbe** inhibits other **microbes**. Both are chemical competition.

**Interspecific competition** leading to **competitive exclusion** — not predation or disease.

If their niches **overlap only partly**, they can use slightly different resources and avoid full competitive exclusion.

A species with a **disproportionately large effect** on its community relative to its **abundance** — remove it and the community structure changes dramatically.

The **keystone of an arch** — the small top stone that holds the arch up; remove it and the **whole arch collapses**.

**No** — a keystone species is often present in **small numbers**; its importance comes from **what it does**, not how common it is.

A predator that controls the **strongest competitor**, keeping its numbers down so **many other species can coexist** — raising biodiversity.

A keystone species that physically **changes the habitat** (e.g. a beaver building a dam), creating conditions many other species depend on.

Its **dams create wetland habitats** that fish, amphibians, insects and birds depend on, so its effect is **far larger than its numbers**.

A chain of **knock-on effects** that spreads through a food web when one species (often a top predator) is added or removed.

Its prey is no longer controlled, so that prey **takes over and out-competes** other species — **biodiversity falls**.

It **raises** biodiversity, by stopping the strongest competitor from taking over so many species can coexist.

It **falls** — without the predator the mussels dominate the rock and crowd other species out.

Keystone predator: a predatory **sea star** eating mussels. Ecosystem engineer: the **beaver** building dams.

Its stabilising **control is removed**, so one species takes over and **crowds out** the rest, leaving fewer different species.